Efficient subnanometric gold-catalyzed hydrogen generation via formic acid decomposition under ambient conditions

Article abstract of DOI:10.1021/ja301696e

Formic acid (FA) has tremendous potential as a safe and convenient source of hydrogen for sustainable chemical synthesis and renewable energy storage, but controlled and efficient dehydrogenation of FA by a robust solid catalyst under ambient conditions constitutes a major challenge. Here, we report that a previously unappreciated combination of subnanometric gold and an acid-tolerant oxide support facilitates the liberation of CO-free H₂ from FA. Applying an ultradispersed gold catalyst comprising TEM-invisible gold subnanoclusters deposited on zirconia to a FA-amine mixture affords turnover frequencies (TOFs) up to 1590 per hour and a turnover number of more than 118-400 at 50 °C. The reaction was accelerated at higher temperatures, but even at room temperature, a significant H₂ evolution (TOFs up to 252 h^-1 after 20 min) can still be obtained. Preliminary mechanistic studies suggest that the reaction is unimolecular in nature and proceeds via a unique amine-assisted formate decomposition mechanism on Au-ZrO₂ interface.

Full text of DOI:10.1021/ja301696e

Article
pubs.acs.org/JACS
Efficient Subnanometric Gold-Catalyzed Hydrogen Generation via
Formic Acid Decomposition under Ambient Conditions
Qing-Yuan Bi, Xian-Long Du, Yong-Mei Liu, Yong Cao,* He-Yong He, and Kang-Nian Fan
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai
200433, P. R. China
S
* Supporting Information
ABSTRACT: Formic acid (FA) has tremendous potential as a safe and
convenient source of hydrogen for sustainable chemical synthesis and
renewable energy storage, but controlled and efficient dehydrogenation of
FA by a robust solid catalyst under ambient conditions constitutes a major
challenge. Here, we report that a previously unappreciated combination of
subnanometric gold and an acid-tolerant oxide support facilitates the
liberation of CO-free H2 from FA. Applying an ultradispersed gold catalyst
comprising TEM-invisible gold subnanoclusters deposited on zirconia to a
FA-amine mixture affords turnover frequencies (TOFs) up to 1590 per
hour and a turnover number of more than 118 400 at 50 °C. The reaction
was accelerated at higher temperatures, but even at room temperature, a significant H2 evolution (TOFs up to 252 h−1 after 20
min) can still be obtained. Preliminary mechanistic studies suggest that the reaction is unimolecular in nature and proceeds via a
unique amine-assisted formate decomposition mechanism on Au−ZrO2 interface.
hydrogen can be efficiently released with a turnover number
(TON) up to 5716 (within 3 h at 40 °C) from FA-amine
mixtures at temperatures as low as 26 °C.6 Moreover, it was
shown that a specifically designed water-soluble Ru-TPPTS
system can release pressurized H2 (up to 750 bar) from
aqueous solution of FA/sodium formate (9:1) at temperatures
of 70−120 °C.7 Most recently, Boddien et al. reported an
elegant noble-metal-free dehydrogenation of FA at 80 °C in
environmentally benign propylene carbonate using an iron-
based molecular catalyst system.8 Compared to the great
progress being made in the FA dehydrogenation with
homogeneous systems,3d,e,6−8 there are scarcely available
reports dealing with the heterogeneously catalyzed dehydrogen-
ation of FA at temperatures below 70 °C.9 Although
decomposition of FA as a model reaction has been extensively
studied over a great number of heterogeneous catalyst systems
since the early 1910s,10 the focus has been largely on the
fundamental reaction mechanism rather than clean and
sustainable H2 generation.9,11 To the best of our knowledge,
few reusable heterogeneous catalysts have been reported that
enable clean and efficient FA dehydrogenation under ambient
conditions.
INTRODUCTION
■
Hydrogen, an essential reagent for chemical and petrochemical
industries, has been considered as one of the best candidates to
satisfy the increasing demands for an efficient and clean energy
supply.1 Controlled storage and release of hydrogen are the
widely known technological barriers in the fuel cell based
hydrogen economy.2 Recently, formic acid (FA), which is one
of the major products formed in biomass processing and also
accessible via a variety of chemical processes based on
hydrolysis of methyl formate or CO2 hydrogenation,3a−c was
identified as a safe and convenient hydrogen carrier in fuel cells
designed for portable use.1b−d,3d,e Hydrogen stored in FA can
be released in situ on demand by catalytic dehydrogenation
(HCOOH → H2 + CO2), with varied low levels of CO,
generally produced by FA dehydration (HCOOH → H2O +
CO), being coliberated as an undesirable byproduct. For the
subsequent conversion of hydrogen into electrical energy, the
CO impurity in the hydrogen gas must be strictly controlled.4
Although FA has been widely used as a cheap and readily
available hydrogen source in liquid-phase transfer reduction
reactions,5 a facile and selective evolution of an ultrapure H2 gas
(CO < 5 ppm) from FA decomposition under ambient
conditions has proven to be extremely difficult. Hence, the
successful development of an excellent protocol that can
facilitate efficient and clean FA dehydrogenation at room
temperature is of high actual interest.
Interest in using supported gold nanoparticles (NPs) or
nanoclusters (NCs) as a low-temperature catalyst for green and
sustainable chemical synthesis has gained considerable
attention over the past few years.12 In parallel to the pioneering
work by Iglesia et al. that well-dispersed Au species can
selectively decompose gaseous FA at 80 °C,10g we recently
In spite of the tremendous research efforts in the field of FA
decomposition for H2 generation, the number of catalysts that
have prominent activity under practical and convenient
conditions is still limited. The most effective catalyst system
developed to date is a Ru−phosphine complex, with which
Received: February 21, 2012
Published: May 8, 2012
© 2012 American Chemical Society
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reported that small Au NPs (approximately 1.8 nm) deposited
on acid-tolerant zirconia (Au/ZrO2) can promote a facile
hydrogen-independent transformation of aqueous bioderived
levulinic acid (LA) and FA streams into γ-valerolactone
(GVL),13 a sustainable and highly versatile intermediate that
has attracted tremendous interest for conversion of lignocellu-
losic biomass into fuels and valuable chemicals.2b,c The essential
role of supported gold is to facilitate rapid and selective
decomposition of FA to produce a hydrogen stream at 120−
180 °C, thereby enabling a highly efficient reduction of LA
without requiring an external source of hydrogen. We have now
discovered a new generation of gold catalyst comprising TEM-
invisible subnanometric gold clusters supported on ZrO2
capable of dehydrogenating a FA-amine mixture under ambient
conditions. This low-temperature FA decomposition method,
using an easily handled and robust heterogeneous gold catalyst
composed of ultrasmall Au NCs dispersed on ZrO2, can make a
significant contribution not only to disclose the intrinsic
catalytic potential of gold but also to establish a more practical
and convenient H2 production process of great industrial
importance.
aspects in facilitating the desired H2 evolution. In particular, the
combination of FA and NEt3 was found to be essential for
achieving high activity in the Au-catalyzed FA dehydrogenation
(Figure S2, Supporting Information). Notably, a 5:2 FA-NEt3
azeotropic mixture (TEAF) gave the best results (Figure S3,
Supporting Information). Thus, at 40 °C, Au/ZrO2 can catalyze
the decomposition of TEAF to produce H2 with an initial TOF
up to 250 h−1 (Table 1, entry 1).
Table 1. Study of Various Solid Catalysts for the Generation
a
of H2 via FA-NEt3 Adducts Decomposition
cat.,T
HCOOH ⎯⎯⎯⎯→ H2 + CO2
NEt3
metal
(μmol)
Vgas
(mL)
TOF
b
c
entry
catalyst
(h−1
)
TON
1
2
Au/ZrO2
30.0
30.0
668
250
912
192 (451)
Au/ZrO2
NCs
2380
741 (1607)
d
3
Au/ZrO2
NCs
30.0
3.75
3.75
992
300
270
252
923
799
245 (670)
748 (1621)
621 (1458)
4
Au/ZrO2
NCs
e
RESULTS AND DISCUSSION
5
Au/ZrO2
NCs
■
Following our previous results in Au-catalyzed FA-mediated LA
conversion,13 we initially examined the decomposition of a
highly acidic 10.5 M aqueous FA solution catalyzed by Au/
ZrO2 at 80 °C. Remarkably, the Au/ZrO2 catalyst was found to
be highly active and selective for FA dehydrogenation in acidic
media, with an initial turnover frequency (TOF) of up to 550
h−1 achievable for the exclusive formation of H2 and CO2
(Table S1, Supporting Information). Prompted by this result,
we examined the gas evolution from aqueous FA in presence of
Au/ZrO2 at 40 °C. However, the reaction turned out to be
extremely slow under this condition (Table S1, Supporting
Information). With the aim to improve catalyst performance,
we examined the use of various inorganic bases in various
ratios, whereby we took into account recent findings by
Laurenczy and Himeda et al.,7,14 which showed a strong
dependence of the catalyst activity and selectivity on the
composition of aqueous FA. After a preliminary screening, we
found that the use of HCOONa with FA/HCOONa ratio of
7:3 led to a significant increase of the reaction rate from 9 to 74
h−1 at 40 °C (Figure S1, Supporting Information). The need
for appreciable amounts of formate was in line with previous
observations that the crucial formate dehydrogenation step
during the transition-metal-catalyzed FA decomposition is
highly sensitive to the concentration of formate ions.7,14
f
6
Ag@Pd/C
Au/Al2O3
Pd−Au/C
Au/TiO2
30.0
30.0
30.0
30.0
3.75
80
49
64
38 (56)
53 (117)
69 (102)
175 (382)
406 (908)
g
7
174
146
566
168
h
8
90
9
226
559
10
Au/TiO2
NCs
11
12
Au/SiO2
Au/C
30.0
30.0
3.75
0
0




i
13
Au/ZrO2
NCs
553
1593
1318 (2990)
a
Reaction conditions: 5.0 mL scale of adducts (53.0 mmol FA, 21.2
b
mmol NEt3), 40 °C, reaction time 3 h. Initial TOF after 20 min.
c
TON for 1 h, numbers in parentheses refer to TON for 3 h.
d
e
Reaction performed at 25 °C. The seventh reuse of Au/ZrO2 NCs
f
g
catalyst. Ag@Pd/C prepared according to ref 9. Au/Al2O3 prepared
h
according to ref 10g. Pd−Au/C prepared according to ref 11a.
i
Reaction performed at 50 °C.
Bearing in mind that particle size has proven especially
influential for supported gold catalysts, we envisioned that
dispersed Au NCs with improved metal dispersion could be
more effective for H2 production from TEAF. To explore this
possibility, a new heterogeneous Au catalyst sample composed
of ultrasmall Au NCs finely dispersed on ZrO2 (denoted as Au/
ZrO2 NCs) was prepared by a careful regulation of the
preparation conditions (see the Supporting Information for
experimental details). To our delight, this catalyst gave much
higher TOFs: up to 748 and 540 h−1 after 1 and 3 h at 40 °C
(Table 1, entry 4), respectively. Particularly noteworthy is that
this Au catalyst gave an exceptional initial TOF up to 923 h−1,
which constitutes the highest activity ever reported for
heterogeneously catalyzed FA decomposition. At temperature
as low as 25 °C, Au/ZrO2 NCs can also decompose FA with
initial TOF of ca. 252 h−1 (Table 1, entry 3). Moreover, a FA
conversion of up to 94% can be readily obtained, even at 25 °C
(Figure 1). Also of note is that in all catalytic experiments, only
an ultratrace level of CO (<5 ppm, Figure S4 and Table S2,
Supporting Information) has been detected in the evolved
gaseous mixture. Thus, FA can be easily and efficiently
To optimize the reaction further, we turned our attention to
the decomposition of FA and amine adducts, a useful
compound material that has received tremendous attention in
recent years as an efficient low-temperature hydrogen
generation system wherein several homogeneous Ru- or Fe-
based complexes bearing air-sensitive phosphine or terpyridine
ligands were employed as the catalysts.6,15 It should be
emphasized that no reusable solid catalyst has yet been
reported to be active and selective for this particular system at
low temperature, despite the anticipated advantages in catalyst
separation and processing. We were pleased to find that an
efficient decomposition of a FA-amine mixture can be achieved
by using the easily handled ligand-free solid Au/ZrO2 catalyst at
40 °C. Additional experiments with various tertiary alkyl amines
and varying ratios of amines to FA showed that the
composition of the reaction medium was one of the key
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activation energy (Ea) of this process was estimated to be 49.3
kJ mol−1 (Figure S6, Supporting Information), which is lower
than most of the previously reported heterogeneous catalyst
systems for FA dehydrogenation.10g Furthermore, the reaction
appears to be wholly unimolecular since decomposition of
either HCOOD or DCOOH under identical conditions leads
to exclusive formation of HD rather than a mixture of H2, HD,
and D2 (Table 2). The most significant finding, however, was
Table 2. Rate of H2 Evolution, KIEs, and Isotope
Distribution for Evolved Hydrogen Gas in Decomposition of
a
FA Catalyzed by Au/ZrO2 NCs
fraction (mol %)
rate (mmol min−1
b
−1
entry
FA
gAu
)
KIE
H2
D2
HD
Figure 1. Kinetic traces for gas evolution via FA dehydrogenation
catalyzed by Au/ZrO2 NCs. Reaction conditions: 5.0 mL scale of
TEAF adducts (53.0 mmol FA, 21.2 mmol NEt3), 30.0 μmol Au.
1
2
3
4
HCOOH
HCOOD
DCOOH
DCOOD
52.1
30.6
20.0
11.6

100
0
0
0
0
1.7
2.6
4.5
100
100
0
0
0
0
100
converted into CO-free H2-containing stream by ultradispersed
Au, which is important for fuel cell applications.
a
Reaction conditions: 5.0 mL scale of adducts (53.0 mmol FA, 21.2
mmol NEt3), 3.75 μmol Au, 40 °C, reaction time 2 h. KIE = rate
b
The reaction did not proceed at all with Au-free ZrO2 (Table
S2, Supporting Information), showing that the presence of gold
was essential for achieving high activity in the dehydrogenation
of FA in TEAF. A comparison with palladium, platinum,
ruthenium, rhodium, and iridium NPs supported on zirconia or
activated carbon as reference catalysts showed that gold is
uniquely active for selective FA decomposition compared with
other noble metals (Table S2, Supporting Information). Note
that previously reported solid catalysts, such as Ag@Pd/C,9
Au/Al2O3,10g and Pd−Au/C11a showed only very low activities
under the conditions as employed in this work (Table 1, entries
6−8). Since it has been shown that the choice of the underlying
support plays an important role in gold catalysis, we also
evaluated gold NPs supported on TiO2, and the resultant
catalyst was significantly less active for FA decomposition under
similar reaction conditions (Table 1, entry 9). Meanwhile, gold
supported on silica (Au/SiO2) and activated carbon (Au/C)
was completely ineffective for the desired reaction (Table 1,
entries 11 and 12). These results indicate that both the noble
metal and the nature of the support play important roles in the
catalytic activity of FA decomposition. Among the various
catalysts investigated in this study, ultrasmall Au NCs deposited
on ZrO2 (Au/ZrO2 NCs) afford by far the best catalytic
performance.
(entry)/rate (entry n) (n = 2−4).
that the present subnanometric Au-catalyzed FA dehydrogen-
ation can proceed smoothly even at elevated pressures. The
pressure of evolved gas may exceed 5.3 MPa (Figure S7,
Supporting Information), with the FA decomposition being
allowed to go to completion in a separate reaction performed in
a closed vessel. This fact shows that the pressures in the system
did not inhibit the reaction.
Having established that Au/ZrO2 NCs are able to catalyze
the generation of pressurized hydrogen at ambient conditions,
we have explored the possibility to produce a continuous
stream of practical applications by using the Au/ZrO2 NCs-
mediated FA dehydrogenation methodology. We have
constructed a high-pressure device that allows a continuous
addition of FA and the control of gas outflow under pressurized
conditions. By maintaining a constant pressure of ca. 2.0 MPa
inside the autoclave, the activity of the Au/ZrO2 NCs sample at
50 °C could remain constant for more than 100 h, in which the
TON approached up to 118 400 with an excellent average TOF
of approximately 1184 h−1 (Figure 2). Under continuous
hydrogen production conditions as shown in Figure 2, the
average rate of H2 gas production was determined to be ca. 148
The high catalytic activity of Au/ZrO2 NCs prompted a
more detailed study, and several important features of the
subnanometric Au-mediated hydrogen generation from FA
have been elucidated. One notable advantage is that the
hydrogen evolution rate at 40 °C did not depend on the O2
concentration in the reaction atmosphere (Table S2,
Supporting Information), thus offering practical and real
advantages over existing technology or methods to establish a
more convenient, readily available H2 production process for
micro fuel cell applications. Moreover, on addition of Au/ZrO2
NCs into the TEAF mixture, the H2 gas can be immediately
generated without any induction period, in contrast to
previously reported activities using Ru-based complexes.6 In
addition, the Au/ZrO2 NCs catalyst was recoverable by simple
filtration and proven robust even after seven reuses (Table 1,
entry 5, and Figure S5, Supporting Information).16 It was also
found that the hydrogen evolution rate can be greatly enhanced
at 50 °C (Table 1, entry 13, and Figure 1). The apparent
Figure 2. Continuous H2 gas production using the Au/ZrO2 NCs
TEAF system. Reaction conditions: 10.0 mL scale of adducts (106.0
mmol FA, 42.4 mmol NEt3), 7.5 μmol Au, 50 °C.
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L H2 h−1 gAu−1. Such hydrogen output corresponds to a
the DR UV−vis data (Figure S10, Supporting Information)
unambiguously demonstrate that the gold dispersion in the Au/
ZrO2 NCs sample is distinctly higher than that for Au/ZrO2,
the identification of the ultrasmall Au clusters by TEM analysis
was unsuccessful (Figure 3) for the Au/ZrO2 NCs sample,
apparently owing to the very fine nature of the Au species.
Extended X-ray absorption fine structure (EXAFS) measure-
ments have thus been carried out to clarify a possible structure
of the metallic Au0 species. However, no meaningful
information can be obtained (not shown), possibly due to
the poor contrast between gold and ZrO2 support (in
transmission mode) or the strong interference of fluorescent
signal of ZrO2 support (in fluorescent mode).21 We then
sought to use a low-temperature CO titration approach22 to
elucidate the intricate nature of the TEM-invisible Au species.
Thanks to the method of pulse flow CO chemisorption at −116
°C, it has been possible to clarify the subnano nature of the Au
species (approximately 0.8 nm, Table S4, see the Supporting
Information for analysis details) through a careful quantitative
estimation.
−1
theoretical power density of 200 W h−1 gAu for energy
generation.6a,9,11a,17 Taking an operation efficiency of 60% and
a typical energy requirement value of ca. 0.5−2.0 Wh for
portable terminals, such as a cell phone in consideration, it
turns out that 0.5−2.0 g of the present Au/ZrO2 NCs catalyst
would be sufficient to supply hydrogen for small proton
exchange membrane (PEM) fuel cell devices.18
Although extensive mechanistic study has not yet been
conducted, the decomposition of the transient Au-formate
species formed by critical FA dissociation could be involved in
the rate-determining step on the basis of kinetic isotope effect
(KIE) measurements with HCOOD, DCOOH, and DCOOD
(Table 2) as well as the indispensable role of NEt3 in facilitating
the reaction progress (Figure S3, Supporting Information). As
depicted in Scheme 1, the key aspect of NEt3 is to facilitate the
Scheme 1. Possible reaction pathway for hydrogen evolution
from the TEAF system over the Au/ZrO2 NCs catalyst
These results, together with the fact that markedly improved
H2 evolution can also be achieved with very small gold
(approximately 1.1 nm) dispersed on titania (Table 1, entry
10), highlight the pivotal role of the subnanometric nature of
Au clusters in facilitating the title reaction. It should be noted
that such subnanoscale phenomenon is consistent with a
previous observation by Iglesia et al.10g that highly dispersed
TEM-invisible gold species on alumina show superior catalytic
activity toward gas-phase FA dehydrogenation at 80 °C. To
gain further information about the effect of Au particle size on
catalytic activity, we compared Au/ZrO2 NCs and Au/ZrO2 for
the room temperature (RT) CO oxidation, low-temperature
(LT) water−gas shift (WGS) reaction and the title reaction
(Figure 4). It is revealed that both the FA dehydrogenation and
WGS reaction over Au/ZrO2 NCs proceed much more rapidly
than those for the Au/ZrO2 sample. This is not unexpected
given the above mechanistic study and generally accepted
fact10g that a common formate-type intermediate could be
involved for both FA dehydrogenation and LT-WGS reactions.
The above results are however in sharp contrast to those that
occurred for ambient CO oxidation, the most widely reaction
investigated for supported gold catalysts. It turns out that the
Au/ZrO2 NCs sample can only deliver a very limited activity
for RT CO oxidation, as opposed to the much higher activity as
identified for the ZrO2 supported Au NPs having diameters of
ca. 1.8 nm.23 It should be mentioned here that although the
particle-size effect of Au has been extensively studied for CO
oxidation, most studies have focused on the catalysts with a
gold size above 2 nm.24 To our knowledge, these results
constituted the first and clear evidence of the reaction-
dependent particle size effect in supported Au catalysts.
O−H bond cleavage as a proton scavenger, leading to the
formation of a metal-formate species during the initial step of
the reaction. The Au−formate complex may undergo further
dehydrogenation, in association with the +HNEt3 species
formed via the reaction of NEt3 with FA, to produce molecular
hydrogen along with CO2 via a β-elimination pathway.10g,19
Meanwhile, the exclusive formation of HD from HCOOD or
DCOOH indicates that the final hydrogen desorption step is
irreversible and also kinetically relevant, highlighting the
feasibility of using renewable FA as a convenient in situ
hydrogen source in place of molecular H2 for sustainable and
green organic synthesis. In the overall reaction for FA
dehydrogenation, the cooperative action between ultrasmall
Au clusters and ZrO2 matrix plays an important role in leading
to the crucial Au−formate activation under mild conditions.
A full characterization of the Au/ZrO2 NCs sample was
performed. From the X-ray photoelectron spectroscopic (XPS)
data presented in Figure S8, Supporting Information, it can be
seen that there is only metallic gold on the surface of the Au/
ZrO2 NCs catalysts.20 The fact that no bands for Au+ or Auδ+
can be seen from the diffuse reflectance infrared Fourier
transform (DRIFT) spectra (Figure S9, Supporting Informa-
tion) of adsorbed CO further confirm the exclusive presence of
metallic Au species in this material. Whereas the surface Au/Zr
atomic ratio (Table S3, Supporting Information) together with
A further noteworthy finding of the present work is that apart
from the gold particle size, the identity of the underlying
support also plays an essential role in determining the catalytic
performance of the final materials. Specifically, the combination
of small size gold and ZrO2 is found to be crucial to give an
exceptionally active catalyst capable of facilitating the liberation
of CO-free H2 from FA under ambient conditions. Zirconia is a
unique material of excellent thermal stability and chemical
inertness, moreover, its surface possesses both acid−basic and
redox functions.25 As a result, ZrO2 has attracted considerable
recent attention as a support material as well as a catalyst. For a
number of reactions,26 the use of ZrO2 as a support has been
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Figure 3. TEM images of (a) Au/ZrO2 (with selected Au nanoparticles marked with arrows), (b) the gold particle size distribution on Au/ZrO2, and
(c) fresh and (d) used Au/ZrO2 NCs.
catalysts, which shows an excellent correlation with the catalytic
data as reported in Table 1. Therefore, in line with the
preliminary mechanistic insight as extracted from KIE studies,
the fact that the amphoteric zirconia support can substantially
facilitate crucial FA deprotonation appears to be a key factor for
achieving high activity in the catalytic FA dehydrogenation.
CONCLUSIONS
■
In summary, for the first time, mild and selective dehydrogen-
ation of FA/amine mixtures was demonstrated using ultra-
dispersed subnanometric gold as catalysts. The reactions,
catalyzed by Au subnanoclusters dispersed on acid-tolerant
ZrO2, proceed efficiently and selectively under ambient
conditions, without the generation of any unwanted byproduct,
such as CO, and with high TOFs/TONs. Moreover, the
reactions proceed very well also at elevated pressures, as
demonstrated for the cases of the continuous generation of a
pressurized stream for H2-based power generation, representing
a significant step toward the practical development of formic
acid as a viable storage medium for hydrogen that could be
used in fuel cells or chemical synthesis. A unimolecular
mechanism involving unique amine-assisted formate decom-
position at the Au−ZrO2 interface is supported by the exclusive
formation of HD and the primary kinetic isotope effects
measured from HCOOD or DCOOH dehydrogenation.
Whereas the exceptional activity of subnanometric Au toward
FA activation promises a new area of gold research, we
Figure 4. Comparison of reaction rate of Au/ZrO2 and Au/ZrO2 NCs
for room temperature CO oxidation, LT-WGS and FA dehydrogen-
ation (5.0 mL scale of adducts with 53.0 mmol FA and 21.2 mmol
NEt3, 3.75 μmol Au, 40 °C, reaction time 2 h). Reaction rate based on
total Au.
reported to give improved activity and selectivity because of the
amphoteric character of its surface nature. To clarify the origin
of the enhanced FA decomposition activity achieved by using
ZrO2 as support, we have examined the effect of support on the
interaction between HCOOD and several catalysts with similar
mean Au particle sizes (ca. 2 nm) using time-resolved in situ IR
(Figure 5). It is revealed that the H/D exchange reaction over
Au/ZrO2 occurred with much higher rates than that over other
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(ZrOCl2·8H2O) was dissolved in 200 mL deionized water, and the pH
was adjusted to about 9.5 by dropwise addition of NH4OH (2.5 M)
under stirring at room temperature. The resultant hydrogel was
washed with deionized water until free of chloride ions after stirring for
8 h. The precipitate was then dried at 100 °C overnight followed by
calcination at 400 °C in air for 2 h to obtain the final support.
Preparation of Au/ZrO2 Catalyst. A modified deposition−
precipitation (DP) procedure has been employed to prepare the Au/
ZrO2 sample.13 Briefly, 2.0 g ZrO2 powder was dissolved with desired
amounts of aqueous solution of HAuCl4 (100 mL, 1 mM), and the pH
was adjusted to 9.0 by dropwise addition of 0.25 M NH4OH
(CAUTION: the addition of NH4OH to HAuCl4 solution could give
rise to highly explosive fulminating gold). After 6 h of stirring at room
temperature, the catalyst was washed five times with deionized water
and separated by filtration. The samples were dried at 110 °C in air for
1 h, followed by a careful reduction with a stream of 5 vol% H2/Ar at
350 °C for 2 h. The resulting Au/ZrO2 sample has a specific surface
area of ca. 113 m2 g−1 and a gold concentration of 0.8% by weight.
Preparation of Au/ZrO2 NCs Catalyst. Results from previous
studies27 show that the size of the gold particles, an essential
requirement for attaining active catalysts, is critically dependent on a
number of parameters including the preparation methods, the choice
of support, and, in particular, the conditions used for catalyst
preparation. Focusing on the preparation parameters that influence
the Au particle size in Au/ZrO2 samples prepared by the modified DP
procedure as described above, we have been able to prepare a new
generation of Au catalyst with a much smaller gold size than that of the
Au/ZrO2 catalyst by decreasing the initial concentration of the
HAuCl4 stock solution and an optimization of the thermal activation
conditions. Basically, an appropriate amount of ZrO2 powder was
added to the aqueous solution of HAuCl4 (0.25 mM),and the pH of
which was adjusted to 9.0 with 0.25 M NH4OH under stirring. The
aqueous dispersion was stirred for 8 h at 25 °C, while the pH was
maintained constantly on 9.0 and then filtered. Extensive washing with
deionized water was then followed until it was free of chloride ions.
The sample was dried under vacuum at 25 °C for 12 h, followed by a
careful reduction with a stream of 5 vol% H2/Ar at 250 °C for 2 h. The
resulting sample, containing 0.8 wt % Au as determined by ICP-AES, is
denoted as Au/ZrO2 NCs, which has a specific surface area of ca. 114
m2 g−1.
Decomposition of FA in an Open System. All catalytic
experiments were carried out under ambient atmosphere of air. It
should be noted that the use of inert atmosphere (N2) gave essentially
the same rate of H2 evolution as air in this reaction. All reactions,
unless otherwise stated, were performed in a double-walled thermo-
statically controlled reaction vessel (10 mL) under steady magnetic
stirring (800 rpm) at a given temperature (25−50 °C) with a reflux
condenser, which is connected to an automatic gas buret, where the
gases are collected (temperature kept constant at 25 °C during
measurements). The gas buret is equipped with a pressure sensor.
Evolving gas during the reaction causes a pressure increase in the
closed system, which is compensated by volume increase of the buret
syringe by an automatic controlling unit. The gas evolution curves are
collected by a personal computer. There is a two piston buret for
measurements above 100 mL. In addition, evolved gas was
qualitatively and quantitatively analyzed by GC (Agilent 6820
equipped with a TDX-01 column connected to a thermal conductivity
detector). Typically a ratio of hydrogen and carbon dioxide of 1:1
( 5%) is detected.
Reuse of Au/ZrO2 NCs Catalyst for Decomposition of TEAF.
For the catalyst reuse experiment, the centrifuged catalysts from
parallel activity tests were collected and washed with deionized water,
followed by drying under vacuum at room temperature for 12 h and
reduction with a stream of 5 vol% H2/Ar at 250 °C for 2 h. All catalytic
activity tests were carried out by following the same procedure as
described in the Decomposition of FA in an Open System Section. To
verify whether there is any leaching of gold or zirconia during the
catalytic FA decomposition (Table 1, entry 5), the Au/ZrO2 NC was
removed from the reaction mixture by filtration after 3 h reaction.
Figure 5. H/D exchange of HCOOD with surface OH groups of the
catalysts at 60 °C. (a) DRIFT spectra recorded after a 15 min exposure
to 2% HCOOD in He. (b) Integrated intensity of OD band with
respect to exposure time. The fact that no OD band could be found in
the bare ZrO2 sample points to that the relevant FA deprotonation
occurs exclusively at the metal−support interface.
anticipate that by a fine-tuning of the dispersed Au clusters at
the subnano level, it is possible to design a new generation of
advanced nanocatalyst systems that are important for
sustainable energy production and chemical synthesis.
EXPERIMENTAL SECTION
■
Preparation of ZrO2 Support. ZrO2 (specific surface area: 115
m2 g−1, 56% monoclinic phase, and 44% tetragonal phase) powders
were prepared by a conventional precipitation method following the
reported procedure.13 Briefly, 8.0 g zirconium oxychloride octahydrate
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Journal of the American Chemical Society
Article
Analysis of the filtrate by ICP-AES showed no detectable leaching of
Au or Zr (<2.5 ppb) into the solution.
ASSOCIATED CONTENT
■
S
* Supporting Information
Pressurized H2 Generation in a Closed System. The catalytic
experiment was carried out in a Hastelloy-C high-pressure Parr
autoclave (50 mL). Typically, 5.0 mL scale of adducts including 53.0
mmol FA, 21.2 mmol NEt3, and 30.0 μmol Au (Au/ZrO2 NCs), was
placed in the autoclave. Then the autoclave was sealed, and the
internal air was degassed quickly and completely using N2 at room
temperature. The stirrer was started (800 rpm) when the desired
temperature of 40 °C was reached.
Chemicals and materials, catalyst characterization, preparation
of other catalysts, and the supplementary results. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
yongcao@fudan.edu.cn
Continuous H2 Generation Using the Au/ZrO2 NCs TEAF
System. A 50 mL Hastelloy-C high-pressure Parr autoclave with a
modified output valve that allows the control of the gas outflow is
connected to a high-pressure pump (Waters 1525 binary pump for FA
feeding) and to a mass flow controller (MFC, Brooks). Initially a
mixture of 4.0 mL FA and 6.0 mL NEt3 was placed in the autoclave
reactor containing 7.5 μmol Au catalyst (187 mg Au/ZrO2 NCs). The
reaction temperature was controlled internally. The stirrer was started
(800 rpm) until the desired temperature of 50 °C was reached. Once
the initial amount of FA had reacted, continuous addition of FA (ca.
0.34 mL h−1) was started, and the gas was released at a rate (148 L H2
h−1 gAu−1), thus the pressure inside the autoclave could be maintained
at a constant value of ca. 2.0 MPa. As shown in Figure 2, the output
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the National Natural Science Foundation
of China (20873026, 20803012, and 21073042), New Century
Excellent Talents in the University of China (NCET-09-0305),
the State Key Basic Research Program of PRC
(2009CB623506), and Science & Technology Commission of
Shanghai Municipality (08DZ2270500) is kindly acknowl-
edged. The authors are also grateful to Prof. Jian-Qiang Wang
from the Shanghai Synchrotron Radiation Facility, Shanghai
Institute of Applied Physics, Chinese Academy of Sciences, for
the EXAFS analysis.
−1
rate of evolved hydrogen gas was about 1.18 L h−1 gcat at 50 °C
during 100 h. Thus, with 0.5−2.0 g of present Au/ZrO2 NC catalyst,
0.59−2.36 L H2 h−1 can be generated.9 Taking a fuel cell operation
efficiency of ca. 60% into consideration, this gives an energy output
after the small proton exchange membrane (PEM) fuel cell of ∼0.5−
2.0 Wh, which can operate small devices, such as cell phones and toys,
etc.
KIE Experiments. All experiments were carried out by following
the same procedure as described in Decomposition of FA in an Open
System Section. The various deuterium-labeled FA and NEt3 were
purified by anhydrous CuSO4. The isotope distribution for evolved
hydrogen gas was carried out by dividing the reactor effluent into two
parallel streams: one was analyzed with a mass spectrometer (Balzers
OmniStar) and the other with a gas chromatograph Agilent 6820
equipped with a TDX-01 column connected to a thermal conductivity
detector. Typically a ratio of hydrogen (H2 or HD or D2) and carbon
dioxide of 1:1 ( 5%) is detected.
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